Note: Descriptions are shown in the official language in which they were submitted.
~00(?~20
G-3,383 C-4095
GUIDE PATH SHORT DETECTOR
Background of the Invention
This invention relates to a short detector
and particularly to such a detector which detects
shorts between adjacent guide paths. The desire for
factory automation has encouraged the widespread use of
automatic guided vehicles (AGVs) with self contained
motive control systems which travel unattended through
aisles in a factory ~or material delivery. Often the
guidance control consists of signal bearing wires
buried in the floor along the desired AGV path. A slot
or groove in the floor containing the wires is fil}ed
with grout to protect the wire. Each wire or guide
path carries current at a specified frequency and each
AGV has receivers attuned to a given frequency for
delivering position signals to its motive control
system. The AGV then follows the route of a guide path
having the frequency which matches the given frequency
of the AGV. If more than one AGV travels the same
aisle, two or more routes may coincide. It is then
desirable to lay the wires in the same slot. Each wire
carries a different frequency so that each AGV can
follow its respective guide path and ignore the others.
Experience with such systems has shown that
from time to time a sharp piece of metal such as a
screw may fall on the floor, become embedded in the
grout and contact the guide path wiring. Where two
adjacent wires are contacted by the metal intrusion,
the wires are shorted and the signal from one quide
. . ~ - ~ -
,:
... , ............ ~ ., ~ ......... - .
,
~()Q~12C~
path can enter another guide path. In that case, a
current of a certain frequency may divide and follow
two paths so that guide path integrity is lost. An AGV
attuned to that frequency may follow the wrong path,
stop, or exhibit some erratic behavior. This AGV
behavior does not necessarily happen as soon as the
short occurs, and in fact such behavior will not occur
until the vehicle enters a portion of its route which
is affected by the short, so that there will be a time
delay between the shorting event and any noticeable
problem. In any case, the plant operation is
eventually interrupted; and since the AGVs are
unattended, some additional time may elapse before the
condition is noticed. Then maintenance personnel must
analyze any unusual events to determine a cause. Once
it is known that the path is shorted they must locate
and repair the fault. The repair must be made as soon
as possible to prevent or minimize production down
time. Thus it is important to be made aware of a short
as soon as possible. It is also important to know
which of several wires are involved in the short
condition.
The AGV systems often are equipped with
instruments to detect open circuits in the guide paths.
However detectors for short conditions were not
available prior to this invention.
Summary of the Invention
It is therefore an object of the invention to
provide apparatus for detecting shorts in AGV guide
paths. It is another object of the invention to
provide apparatus for identifying specific guide paths
that are shorted.
. :
:. - . . - ~.
~ - - ~ ' -- ;
20V0120
The invention is carried out in a guide path
system having adjacent guide paths with means for
detecting a short condition between guide paths
comprising: current sensing means in spaced locations
along one guide path to generate signals which are
proportional to the currents, means coupled to the
current sensing means for comparing the signals and
producing an output when a current signal difference is
caused by a short condition between the locations, and
means coupled to the amplifier output for responding to
a short condition.
The above and other advantages of the
invention will become more apparent from the following
description taken in conjunction with the accompanying
drawin~gs wherein like references refer to like parts.
~rief Description of the Drawings
Figure 1 is a schematic diagram of a guide
path system incorporating a short detector according to
the invention, and
Figure 2 is a schematic circuit diagram of a
short detector according to the invention.
Descri tion of the Preferred Embodiment
p
The system in which a short detector is useful
is shown schematically in Figure 1. A guide path
generator 10 produces a different frequency for each
path. Such generators are well known. The frequencies
F1, F2 and F3 may be 2575 Hz, 3433 Hz and 4390 Hz, for
example. The current supplied to each path is
typically 0.5 to 1 amp.
A first path 12 traces a closed circuit along
the desired route and is connected by a source line 16
and a return line 18 to the generator 10 which supplies
.. - . . . .
.
.~ ~ ` . ' -
2C~0Q120
current at a frequency Fl to the path. A short
detection relay 20 is connected at or near the
generator 10 and has terminals T1 and T2 connected
serially in the source line 16 and terminals T3 and T4
in the return line 18. A second path 22 is similar to
the path 12 and the two paths have a common route C
over a portion of their range. The path 22 is
connected by source and return lines 26 and 28 to the
generator to receive current at a second frequency F2
and another short detection relay 24 is coupled into
the lines 26 and 28. ~ach short detection relay 20, 24
is connected to an annunciator 30, 32 respectively for
giving a warning or indication when a short is
- detected.
While it is usually most convenient to couple
the detector relays to the paths at the generator, it
is also effective to place them elsewhere in the path
so long as the terminals T2 and T4 are connected to
spaced points along the wire to embrace the common
route C where a short can occur.
The short detection relay relies on the
presence of current in one path which originated in
another path. Thus by comparing the currents at two
spaced locations in a path it is possible to detect any
extraneous current which leaked in ~rom another path at
a point between the two spaced locations. For the
system where each path has a different irequency a
small amount of leakage can be detected.
Details of the short detection relay 20 are
shown in Figure 2. A first transformer 34 has a
primary winding 36 connected across terminals Tl and T2
and a secondary winding 38 grounded at one end and
: , .
.~ ~,. : ' . .
~ .
2~00121~
connected at the other end to an input of an
operational amplifier 40 via a re~istor 42. A second
transformer 44, identical to the first transformer, has
a primary winding 46 connected across terminals T3 and
T4 and a secondary winding 48 grounded at one end and
connected at the other end to the input of the
operational amplifier 40 via a resistor 50. The
resistors 42 and 50 have nominally equal value (56K).
It is important that the transformer secondary windings
38, 48 be connected to the amplifier 40 in 180 degree
phase opposition so that equal currents will cancel out
to produce a null input to the amplifier so long as the
same current flows in the two primary windings 36, 46.
In the event of transformer inequalities, one of the
resistors 42 or 50 can be adjusted to compensate for
any resulting offset to assure the null input.
The amplifier 40 has a second input coupled
through a resistor 52 (18K) to ground to establish a
reference voltage to bias the amplifier off at very low
signal voltages. A feedback resistor 54 (lM) couples
the output and the first input terminal. The amplifier
40 output is fed through a resistor 56 (18K) to a first
input of another operational amplifier 58 which has a
second input connected to an adjustable tap of a
potentiometer 60. The operational amplifier 58 and its
attendant circuitry comprise a modified zero crossing
detector 61. The potentiometer biases the amplifier 58
on to provide a positive output for low voltage signals
on the first input. Adjustment of the potentiometer
will adiust the sensitivity of the detector. A zener
diode 62 forms the feedback path for the amplifier 58
and is poled to make the output go to +0.7 volts
:
.
2~3~ 1Z(~
whenever the input is zero or less and -4.9 volts
whenever the input is greater than zero.
A rectifier and filter comprise a diode 64
with its cathode connected to the amplifier 58 output
and its anode connected to the gate of an FET 66 and to
ground through a capacitor 68 (0.33uF). A resistor 70
(150X) across the diode passes a positive voltage to
the gate at steady state conditions to hold the FET on.
A relay 72 in series with the FET 66 is energized when
the FET is on. A diode 74 across the relay 72 bypasses
inductive spikes when the relay is turned off. An LED
76 in series with a resistor 78 ~540) is connected
across the relay to give a visual signal when the FET
is energized.
In operation, the relay 72 is arranged to
activate an annunciator when the relay turns off. Thus
when there is no short the relay 72 and the FET 66 are
turned on and when a short is detected they are turned
off so that the annunciator issues a signal. During
normal operation of the guide paths the transformers
will have equal input currents and equal output
voltages. The output voltages are in opposite phase
and cancel out so that the net transformer output will
be substantially zero. The zero net output effects a
ground or zero voltage at the output of the amplifier
40 which, in turn, causes the zero crossing detector 61
to have a positive steady state output voltage which is
applied through the resistor 70 to the FET to thereby
turn on the EET and the relay 72.
In the event of a short either of two
conditions may occur: 1) the single frequency signal of
a guide path becomes weaker at the return line, or 2) a
20~)01.2~
second frequency appears on the return line. In either
case the normal sin~le frequency signal on the source
line does not balance out the signal on the return
line. The signal difference will appear in the net
transformer output which is applied to the first
amplifier 40 making its input and output alternate
between positive and negative values. This causes the
zero crossing detector 61 to oscillate between +0.7 and
-4.9 volts. The diode 64 readily discharges the
capacitor 68 to a negative voltage which turns off the
FET 66 and the relay 72, thus activating the
annunciator. The filtering action of the capacitor
holds a negative voltage on the FET gate as long as the
oscillation continues.
It will thus be seen that the short detector
provides immediate indication of a short between guide
paths embedded in the same slot. Tests of the device
show that a current imbalance of 6% or 7% is detected
and it is effective when the short is from the source
side to a return side of a path, from source side to
source side, or even return to return sides when the
returns are connected to a common ground. Thus the
detector is sensitive to very small amounts of current
imbalance so that even if a short has high impedance it
may be detected and corrected before it becomes a
problem to the AGV. When each guide path has a short
detector the information from each shorted path reveals
which paths are shorted; this information along with a
map of the guide path routes narrows down the location
of the short.
. .
: - ' , ,.' ' :
.'' ,, ~ .
